1. Field of the Invention
The present invention relates to high numerical aperture and immersion lithography.
2. Related Art
Lithographic tools and techniques are increasingly called upon to print patterns at a high resolution. For example, in the manufacture of semiconductor dies or chips, patterns of circuit features, such as lines, contact holes, or other elements, often need to be printed at a high resolution to improve the packing density of circuit elements and reduce the pitch of the pattern. Certain circuit features, such as contact holes or vias, are especially difficult to fabricate.
A well-known parameter relating to lithography resolution is the critical dimension (CD). The CD is the size of the smallest geometrical features which can be formed during semiconductor device and circuit manufacturing using a given technology. The critical dimension can be described as shown in the following function:CD=k(λ/NA),where λ is a wavelength used in lithography, NA is the numeric aperture, and k is the dielectric constant. Among the trends in lithography is to reduce the CD by lowering the wavelength used, increasing the numeric aperture, and reducing the value k.
Printing can be difficult in low k applications. For example, contact holes are difficult to print when k is less than 0.5. A high-contrast image of sufficient quality that includes groups of contacts holes like contact arrays is especially hard to print.
Techniques to enhance contrast using a very high NA and off-axis illumination have been used but these techniques fail for small pitches. For example, at 157 nm wavelength, 0.93 NA, the limiting pitch (based on resolution) is roughly 135 nm (k=0.4)—which is too high for certain applications. Also, a forbidden pitch may occur. This means that if the illumination is optimized for a given pitch, printing other pitches simultaneously may become impossible. Forbidden pitch can be manifested in a low normalized image log slope (NILS) or poor CD control for the forbidden pitch.